Cordless power tools provide many advantages to traditional corded power tools. In particular, cordless tools provide unmatched convenience and portability. An operator can use a cordless power tool anywhere and anytime, regardless of the availability of a power supply. In addition, cordless power tools provide increased safety and reliability because there is no cumbersome cord to maneuver around while working on the job, and no risk of accidently cutting a cord in a hazardous work area.
However, conventional cordless power tools still have their disadvantages. Typically, cordless power tools provide far less power as compared to their corded counterparts. Today, operators desire power tools that provide the same benefits of convenience and portability, while also providing similar performance as corded power tools.
Brushless DC (BLDC) motors have been used in recent years in various cordless power tools. While BLDC motors provide many size and power output advantages over universal and permanent magnet DC motors, it is always desired to manufacture more compact motors while providing the same or higher power output.
BLDC motors are available as canned motors, where all the motor components are securely assembled inside a cylindrical motor can or motor housing. The motor housing includes piloting features for the rotor end bearings to retain the rotor assembly securely within the stator. The motor housing is encapsulated inside a power tool via two power tool housing halves.
Alternatively, BLDC motors may be without a motor housing or can, where the stator/rotor assemblies are mounted directly inside the power tool. Such motors are typically provided with two end bearing support mounts provided at the two ends of the stator assembly. The bearing support mounts are axially fastened together on the stator via screws located on the outer surface of the stator. The bearing support mounts constraint the axial movement of the rotor within the stator. The bearing support mounts also typically include radial retention features, for example radial constraints that partially wrap around the outer surface of the stator, to constraint the radial movement of the rotor within the stator. Radial retention features have to be manufactured with great precision to ensure that an air gap is provided between the rotor and the inner surface of the stator.
U.S. patent application Ser. No. 13/919,352 (Publication No. 2013/0270934), which is incorporated herein by reference in its entirety, describes an example of a BLDC motor without a motor housing. As shown in FIGS. 2A and 2B of this disclosure, the two bearing support members (i.e., a ring gear mount and a hall board mount assembly, also commonly referred to as motor caps) and the stator all include fastener receptacles that allow the three components to be securely fastened together. Additionally, the two bearing support members include piloting and retention semi-cylindrical walls that partially cover the outer diameter (OC) of the stator lamination stack. These features radially retain the two bearing support members, and consequently the rotor assembly, with respect to the stator.
While these fastening and piloting features are important in precise and secure assembly of the rotor with respect to the stator, they add to the overall outer diameter of the motor. In particular, the piloting and retention walls add to the diameter of the stator lamination stack. Also, the screws receptacles add to the outer diameter of the stator and the two bearing support members. In BLDC motors, particularly in handheld portable power tools where space is limited, it would be greatly desirable to construct these piloting and retention features in a way that does not affect the length and diameter of the motor.